I cant answer your question, but take a look at the example configs that come with mysql. Theres configs for small, large and huge.
If the huge config (referring to database size and hardware) is using a large value, then its safe to assume its not OTT )

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Thanks buddy, the huge-innodb config only has 4Gb of RAM, so it's hard to know how things relate on a system with 4x the RAM. E.g. is it simply linear? For now I've set each log file at 1Gb, with a combined of 2Gb. My gut feeling is that this is high enough.

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The 25% hint is IMHO related to older setups (not everybody can afford 16G RAM in a server )) So virtually having files > to 1GB would cause recovery and shutdown times to be extremely slow and inefficient.